(1) Field of the Invention
The present invention relates to transmission devices, and more particularly, to a transmission device that performs a transmission control on a ring network.
(2) Description of the Related Art
As is known, SDH/SONET has been standardized as a signal hierarchy multiplexing system in digital transmission, and development of economical digital networks has been progressed. A BLSR (Bi-directional Line-Switched Ring) has been widely employed as a ring-shaped network configured by SONET nodes. The BLSR is a ring network in which each route between nodes is doubled in two ways, and if a fault occurs in one of the routes in two ways, the traffic flow is switched to the other route.
For a BLSR involved in OC-48 (2.488320 Gb/s) consisting of CH1 through CH48, CH1 through CH24 are assigned to the working path group, and CH24 through CH48 are assigned to the protection path group. That is, 24 channels are assigned to each path group, and half of the channels is ensured for protection.
A fault is bypassed in the BLSR as follows. If a fault occurs in a line including the working path group, all of the 24 channels of the working path group is forcedly replaced by the channels of the protection path group in the reverse direction. Even when some channels of the protection path group is used for communications, these channels are used to save the working path group.
The recent technology focuses on a fault restoration control called NUT (Non-preemptible Unprotected Traffic). By setting NUT, it is possible to avoid using some channels for restoration. In the above-mentioned example, if CH2 is set as NUT, CH2 is not switched for restoration. There are two types of NUT, a basic NUT and an enhanced NUT.
FIG. 21 is a diagram for explaining the basic NUT. Nodes 101-104 configure a BLSR network. Routes that connect two adjacent nodes among nodes 101-104 are spans Sp1-Sp4. In the basic NUT, NUT is set to a pair of channels in the working and protection path groups. NUT is also set to all spans of the ring network. For example, when CH1 in the working path group is placed out of a channel used for restoration, CH25 in the protection path group is also placed out of a channel for restoration. NUT is set to all of the spans Sp1-Sp4.
FIG. 22 is a diagram for exampling the enhanced NUT. NUT is set, on the span basis, to each of the individual channels in the working path group and each of the individual channels in the protection path group. For example, NUT may be set to only span Sp1 so that CH1 in the working path group is placed out of a channel used for restoration, and may be set to only span Sp2 so that CH48 in the protection path group is placed out of a channel for restoration.
However, the conventional NUT setting control cannot set NUT efficiently and easily because the operator is needed to provide each node with an instruction for setting NUT. In other words, the conventional NUT setting control does not employ an automatic configuration of setting NUT efficiently in the ring network.
There is also another disadvantage in that, if information used to set NUT is not correctly transferred to each node during establishment of the NUT setting because of a fault or the like, erroneous NUT setting may be established. This may degrade working efficiency and transmission quality.